1. Field of the Invention
The present invention relates to devices employing an absorption cycle, such as absorption-type refrigeration systems. The present invention particularly relates to absorption-type refrigeration systems using a refrigerant, an absorbent, and a heat-transfer additive.
2. Description of Related Art
Absorption-type refrigeration systems typically include an absorber, one or more pumps, one or more generators, a condenser, an evaporator, and necessary piping and controls. The systems use a fluid including absorbent and refrigerant. The fluid is labelled either strong or weak, depending on whether the concentration of absorbent is relatively high or low, respectively. Typically, a weak fluid contains approximately 56-60 weight percent lithium bromide and a strong fluid contains approximately 59-65 weight percent lithium bromide, the exact values depending upon operating temperatures and the design of the cycle.
Operation of absorption-type refrigeration systems can be briefly explained in reference to an absorption cycle using a single generator. A weak fluid exits or is pumped from the absorber. The weak fluid is subsequently applied to a generator, which evaporates refrigerant from the weak fluid. Since evaporating the refrigerant from the weak fluid increases the concentration of absorbent in the fluid, the fluid is now called a strong fluid.
The evaporated refrigerant condenses in the condenser and passes to the evaporator. In the evaporator, the refrigerant absorbs ambient heat, which provides the desired refrigeration effect. The absorbed heat causes the refrigerant to vaporize.
The vaporized refrigerant passes to the absorber, where it is exposed to strong fluid returning from the generator. The strong fluid absorbs the refrigerant, thereby causing the strong fluid to become weak again.
Performance of an absorption-type refrigeration system can be improved by adding certain additives to the system's fluid. For example, addition of 2-ethyl 1-hexanol (octyl alcohol) to an aqueous lithium bromide fluid improves the performance of absorption-type refrigeration systems using such a fluid. More specifically, the additive improves the rate of heat transfer in the absorber and the condenser. Thus, such additives are referred to as heat-transfer additives.
However, in some absorption-type refrigeration systems, especially those employing more than one generator, the operating temperature of a generator may be above the thermal decomposition temperature of the heat-transfer additive. Decomposition of heat-transfer additives adversely affects the performance of absorption-type refrigeration systems, making it undesirable.
Attempts have been made to minimize the decomposition of heat-transfer additives by reducing or eliminating the flow of heat-transfer additives to the generator. For example, U.S. U.S. Pat. No. 4,315,411 describes a separator for separating a part of the heat-transfer additive from the fluid. The separator is disclosed as being at a point where the weak fluid exits the absorber. The disclosed separator operates on gravity separation. It includes a vessel large enough for slowing down the flow velocity of the fluid. The vessel includes an inlet at its lower part and an outlet at its upper part. A small part of the fluid, substantially enriched in heat-transfer additive, flows out of the outlet at its upper part while the bulk of the fluid, weak in heat-transfer additive, flows out of the outlet at its lower part.
A principal disadvantage of this approach is its reliance upon gravity separation through settling, which is believed to be impractical. In particular, such a system is excessively slow. Thus, there is a need for a system that minimizes decomposition of heat-transfer additives without requiring prolonged storage time for the solution.
Performance of some absorption-type refrigeration systems can be improved by adjusting the pumping ratio. The weak solution mass flow to the generator(s) divided by the refrigerant mass flow to the evaporator yields the pumping ratio. Though decreasing the pumping ratio increases the thermal coefficient of performance (heat supplied to the high-temperature generator divided by the cooling produced by the evaporator) in single and double effect refrigeration systems, it also increases the temperature of the generator(s).
Pumping ratio adjustment is considered an undesirable means of improving performance in triple effect systems because of the anticipated increase in generator temperature. The high-temperature generators in triple effect systems typically operate at temperatures that decompose heat-transfer additives. Decreasing the pumping ratio drives the generator temperature even higher. FIG. 4 shows the effect of decreasing the pumping ratio on a 400 tons, parallel connected, triple effect system, such as the system disclosed in U.S. Pat. No. 5,205,136. The temperature of the strong fluid leaving the high-temperature generator increases rapidly as the pumping ratio is decreased. It is believed that the temperature increases because reducing the pumping ratio increases the concentration of strong fluid, which then increases the boiling point of the strong fluid.
Not only does decreasing the pumping ratio adversely affect the heat-transfer additive in parallel connected, triple effect systems, it also does not provide the desired gain in coefficient of performance. The higher temperature strong fluid leaving the generator puts more load on the heat exchangers. Since the heat exchangers recover only approximately 70% of the heat in the strong fluid, the remaining 30% is wasted when it is rejected in the absorber. As the heat of the strong fluid increases, the amount of wasted heat increases.
The increased loss in the heat exchangers negates the coefficient of performance gains realized by reducing the pumping ratio. As shown in FIG. 3, the coefficient of performance of the parallel connected system remains almost constant as the pumping ratio decreases. Thus, adjusting the pumping ratio does not appear to be a viable means of improving the performance of triple effect systems.